Hydrogen is stored. The amount that leaks can be as small as you like. Actual loss as a percentage of what you put in is always negligible. The point often made about leaks is just that H2 must not be allowed to accumulate, so needs positive airflow to prevent detonations.
Mirai is just an electric car with an H2 tank, a fuel cell, and a small battery. The fuel cell has no moving parts: H2 and air go in, power, H2O, and N2 come out. Mirai is very light, so takes much less energy to muscle around than a battery car. Its small battery is to use while the fuel cell heats up, and maybe to absorb some kinetic energy braking.
A battery electric car is much more complex. It needs to manage charge and temperature on all its cells, use braking to recover and store kinetic and downhill energy to try to extend range. The battery is expensive.
But energy is fungible. After there is plenty of renewable generation capacity, hydrogen made from renewables has not cost anybody anything tangible. Until there is plenty of renewables, making hydrogen costs NG, so will not happen much.
Cars will have batteries. Trucks will probably run on ammonia. Aircraft will run on LH2. The kWh that goes into a unit of H2 will not matter much, because it is just sunshine and wind. We need to get there by spending on renewable generating capacity, first.